1. Field of the Invention
The present invention relates generally to multi-piece, golf balls having a solid core made of a foamed composition. Particularly, the dual-layered core has a foam inner core (center) and surrounding thermoset or thermoplastic outer core layer. Preferably, a polyurethane foam composition containing mineral filler particulate, for example, nanoclay particles, is used to form the foam center. The core layers have different hardness gradients and specific gravity values. The ball further includes a cover of at least one layer.
2. Brief Review of the Related Art
Both professional and amateur golfer use multi-piece, solid golf balls today. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The cover surrounds the inner core and may be made of a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas.
In recent years, three-piece, four-piece, and even five-piece balls have become more popular. New manufacturing technologies, lower material costs, and desirable ball playing performance properties have contributed to these multi-piece balls becoming more popular. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a cover of at least one layer to provide a final ball assembly. Different materials can be used to manufacture the core and cover and impart desirable properties to the final ball.
In general, dual-cores comprising an inner core (or center) and a surrounding outer core layer are known in the industry. For example, Sugimoto, U.S. Pat. No. 6,390,935 discloses a three-piece golf ball comprising a core having a center and outer shell and a cover disposed about the core. The specific gravity of the outer shell is greater than the specific gravity of the center. The center has a JIS-C hardness (X) at the center point thereof and a JIS-C hardness (Y) at a surface thereof satisfying the equation: (Y−X)≧8. The core structure (center and outer shell) has a JIS-C hardness (Z) at a surface of 80 or greater. The cover has a Shore D hardness of less than 60.
Endo, U.S. Pat. No. 6,520,872 discloses a three-piece golf ball comprising a center, an intermediate layer formed over the center, and a cover formed over the intermediate layer. The center is preferably made of high-cis polybutadiene rubber; and the intermediate and cover layers are preferably made of an ionomer resin such as an ethylene acid copolymer.
Watanabe, U.S. Pat. No. 7,160,208 discloses a three-piece golf ball comprising a rubber-based inner core; a rubber-based outer core layer; and a polyurethane elastomer-based cover. The inner core layer has a JIS-C hardness of 50 to 85; the outer core layer has a JIS-C hardness of 70 to 90; and the cover has a Shore D hardness of 46 to 55. Also, the inner core has a specific gravity of more than 1.0, and the core outer layer has a specific gravity equal to or greater than that of that of the inner core.
The core sub-structure located inside of the golf ball acts as an engine or spring for the ball. Thus, the composition and construction of the core is a key factor in determining the resiliency and rebounding performance of the ball. In general, the rebounding performance of the ball is determined by calculating its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “Coefficient of Restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under such conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance as opposed to balls with low COR values. These properties are particularly important for long distance shots. For example, balls having high resiliency and COR values tend to travel a far distance when struck by a driver club from a tee.
The durability, spin rate, and feel of the ball also are important properties. In general, the durability of the ball refers to the impact-resistance of the ball. Balls having low durability appear worn and damaged even when such balls are used only for brief time periods. In some instances, the cover may be cracked or torn. The spin rate refers to the ball's rate of rotation after it is hit by a club. Balls having a relatively high spin rate are advantageous for short distance shots made with irons and wedges. Professional and highly skilled amateur golfers can place a back spin more easily on such balls. This helps a player better control the ball and improves shot accuracy and placement. By placing the right amount of spin on the ball, the player can get the ball to stop precisely on the green or place a fade on the ball during approach shots. On the other hand, recreational players who cannot intentionally control the spin of the ball when hitting it with a club are less likely to use high spin balls. For such players, the ball can spin sideways more easily and drift far-off the course, especially if it is hooked or sliced. Meanwhile, the “feel” of the ball generally refers to the sensation that a player experiences when striking the ball with the club and it is a difficult property to quantify. Most players prefer balls having a soft feel, because the player experience a more natural and comfortable sensation when their club face makes contact with these balls. Balls having a softer feel are particularly desirable when making short shots around the green, because the player senses more with such balls. The feel of the ball primarily depends upon the hardness and compression of the ball.
Manufacturers of golf balls are constantly looking to different materials for improving the playing performance and other properties of the ball. For example, Kim et al., U.S. Pat. No. 6,794,447 discloses golf balls incorporating nanofiller material in their cores, outer cover layers, or, intermediate layers. The material includes a polymer such as polyamide, ionomer, polycarbonate, polyurethane, polystyrene, polyethylene, fluoropolymer, polyamide elastomer, thermoplastic polyolefin, polyester elastomer, polyester, polyolefin, thermoplastic elastomer, thermoplastic vulcanizate, or epoxy resin. Nano-particles of inorganic material are dispersed in the polymeric material. The particles have a size of one micron (μm) or smaller and particles preferably consist essentially of clay, such as hydrotalcite, montmorillonite, micafluoride, or octosilicate.
Jordan et al., US Patent Application Publication US 2007/0191526 discloses golf balls having a core and cover surrounding the core. At least one of the layers is formed from a composition comprising nanoclays and a fully neutralized ethylene acid copolymer or other ionomer resin. The acid groups in the ionomer resin may be neutralized by a salt of an organic acid, a cation source, or a suitable base. The nanoclays are described as being high aspect ratio additives typically based on montmorillonite clay. The nanoclays preferably have a 50% average dry particle size of 6 μm or less and a 10% average dry particle size of 2 μm or less with a preferred aspect ratio of 100 to 150.
Sullivan et al., U.S. Pat. No. 7,261,647 discloses golf balls having a core, vapor barrier layer, and cover. The barrier layer is formed from a polymer comprising a nano-materials such as smectite minerals; carbon nanotubes; fullerenes; single-wall and multi-wall carbon nanotubes; polymer nanofibers; and carbon nanofibrils with an average particle size of less than 100 nm. According to Sullivan '647, the nano-material creates a tortuous path across the barrier layer to prevent vapor from penetrating therein. Suitable polymers are described as including ionomeric copolymers and terpolymers, thermoplastic elastomers, polybutadiene rubber, balata, grafted metallocene-catalyzed polymers, polyurethanes, and polyureas.
One drawback with adding mineral fillers, particularly nanometer-sized materials (“nano-materials”), is that such fillers are often difficult to disperse in the composition. The very small size of the nano-particles makes them hard to disperse fully and uniformly. As a result, agglomerates of nano-particles may form in the polymeric matrix. If a significant amount of agglomerates and aggregates form, they may negatively affect physical properties of the composition such as compression strength, flexural modulus, tensile strength, elongation at break, and the like.
Thus, it would be desirable to have a composition containing nano-materials that are dispersed effectively throughout the composition. The nano-materials should be dispersed fully and uniformly to form a homogeneous mixture that can be molded easily into a golf ball core. In addition, the nano-materials should help enhance the physical and mechanical properties of the core. The resulting ball also should have good resiliency since this allows players to generate higher initial ball speed and make shots having greater distance. The present invention provides new foam core constructions having such properties as well as other advantageous features and benefits. The invention also encompasses golf balls containing the improved core constructions.